Alternating current (‘AC’) motor systems typically comprise a motor comprising a rotor and a stator and a motor controller for controlling the voltage and current supplied to drive the motor. In order to ensure good control of the motor, for example in order to meet specified motor performance requirements, the motor controller needs to know the position of the motor rotor.
Various techniques are known for estimating the rotor position. Physical position sensors, such as position and velocity transducers, can be used although such position sensors and their associated cabling and connectors increase the size, weight and complexity of the AC motor system and have also been a source of failure for AC motor systems. In order to eliminate such position sensors, particularly for small low cost motor controllers, much research has taken place into sensorless techniques for determining rotor position for different classes of motors under a variety of different operating conditions.
A simple technique uses the induced back electromotive force (‘EMF’) generated in the motor. However, at rotor standstill or low speed there is insufficient back electromotive force (EMF) generated in the motor to enable an accurate estimate of rotor position.
More complex techniques are based upon injection of appropriate high frequency (‘HF’) reference signals superimposed on the drive torque and flux control signals and the tracking of the response of the stator currents of the AC motor to the injected reference signal in order to determine the rotor position. The frequency of the injected signal is sufficiently higher than the fundamental frequency of the drive currents to be distinguishable from them and is limited by the impedance and reaction time characteristics of the controller. The basis for most low and zero speed sensorless control techniques is the magnitude of a q-axis stator current at the injected signal frequency calculated in rotating direct (‘d-axis’) and quadrature (‘q-axis’) coordinates defined by a d-q reference frame rotating with the rotor, with the d-axis coinciding with the rotor magnetic axis, while the q-axis is perpendicular to the d-axis. This q-axis stator current is generated by the effect of the rotor position on the stator inductance and is referred to as saliency. In a Permanent Magnet (PM) motor, for example, there are several sources of saliencies, such as rotor inherent saliency, saturation based saliency (stator, teeth).
US patent application publication no. 2006/0061319, U.S. Pat. No. 6,894,454, the article “Current Model-Based Sensorless Drives of Salient-Pole PMSM at Low Speed and Standstill” by Ryoji Mizutani et al, in IEEE Transactions on Industry Applications, Vol. 34, NO. 4, July/August 1998 and the article “Initial Rotor Position Estimation of an Interior Permanent-Magnet Synchronous Machine Using Carrier-Frequency Injection Methods” by Yu-seok Jeong et al, in IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 41, NO. 1, JANUARY/FEBRUARY 2005 disclose methods of determining rotor position in which an HF carrier or pulse signal is injected into the stator windings by combining the HF signal with the command voltage signals that control the current provided to the stator of the AC motor to generate magnetic flux component (d-axis) and a torque component (q-axis). The resulting HF components, which carry the saliency position information and which are part of the feedback current from the stator, are then separated from the drive components of the stator current and processed by a processor in the motor controller to determine the rotor position. The feedback current is also fed back as part of a control loop in the controller to control the power applied to the stator.
These known techniques where the command voltage signals are applied to generate the stator drive current simultaneously with the injected HF carrier signal, torque is applied to the rotor before its initial standstill position is known and the rotor moves, delaying and disturbing determination of its estimated position. The initial start-up torque applied to the rotor is reduced by misalignment between the estimated and real initial rotor positions. It is possible to perform a physical alignment sequence, in which the rotor initially moves to a position corresponding to a known position, which is then defined as aligned, but this also involves a delay in applying full initial start-up torque to the rotor.
Furthermore, the Applicant's co-pending international patent application PCT/IB2007/053318 filed 20 Aug. 2007 discloses another sensorless rotor position determining method which avoids interference between the injected carrier signal and HF harmonic components of the command voltage signals generated in the motor due to the operation of the control loop, for example during changes in motor load.